Efficient anti-ice exhaust vent

ABSTRACT

An anti-icing system for a nacelle of a gas turbine powerplant is disclosed. Various construction details have been developed which provide means to exhaust fluid from an inlet shell cavity and into an external medium. In one embodiment, the exhaust means (62) includes a plurality of aerodynamically shaped vanes (66) disposed within the inlet shell cavity (52) and a plurality of exhaust slots (68) wherein each of the vanes is adjacent one of the exhaust slots. The vanes are adapted to turn a portion of the body of fluid flowing within the inlet shell cavity to a direction substantially normal to the direction of flow of the external medium.

TECHNICAL FIELD

The present invention relates to an anti-ice system for a gas turbinepowerplant and, more particularly, to exhaust means for anti-ice fluidflowing through a nacelle cavity.

BACKGROUND ART

A typical aircraft powerplant, such as an axial flow gas turbine engine,includes a nacelle which extends about the engine. The nacelle is theradially outermost structure of the powerplant. The nacelle extendsforward of the gas turbine engine to define an inlet for working fluidto enter the gas turbine engine. The gas turbine engine includes aflowpath in communication with the fluid inlet of the nacelle. Theflowpath extends sequentially through a compressor section, a combustionsection and a turbine section. The compression section and turbinesection include many components susceptible to impact damage, such asrotating blades which extend across the flowpath and interact with theworking fluid.

Safety is a primary concern in the design of powerplants to be used inaircraft applications. One hazard to be avoided is the build-up of iceon the engine and the structure surrounding the engine. The build-up ofice presents many problems. First, ice may add considerable weight tothe engine and to the aircraft. Second, the build-up of ice near theinlet of the nacelle may adversely affect the flow of working fluid intoand through the engine.

Another concern with powerplants is the useful life of the powerplantand components. The build-up of ice near the inlet of the engine maylead to large pieces of ice breaking loose from the inlet and flowinginto the gas turbine engine. Ice flowing into and through the engine maydamage components within the engine, such as the blades, and componentsattached to the nacelle, such as inlet acoustic panels. The damagedcomponents may then require replacement.

Not surprisingly, anti-ice systems for the inlet area of nacelles havebeen the focus of a significant amount of research and developmentwithin the aircraft industry. An example of this is U.S. Pat. No.4,688,745, entitled "Swirl Anti-Ice System" and issued to Rosenthal.This patent discloses a system for injecting hot gases into an annularcavity located on the leading edge of the nacelle. The cavity extendscircumferentially about the inlet. The hot gases are injected into thecavity in a direction tangential to the circumferential direction. Thetangential injection produces a circumferentially flowing body of hotgases within the cavity. Exhaust vents or holes located in a bulkhead ofthe nacelle allow gases within the cavity to escape into the externalmedium flowing around the powerplant. The venting of the anti-ice fluidprevents over pressurization of the cavity in the event of an anti-icefluid flow regulator failure. The exhaust holes lie in a plane parallelto the direction of flow to avoid direct impingement of the body offluid on the exhaust holes.

The anti-ice system disclosed in U.S. Pat. No. 4,688,745 is effective atreducing the built-up of ice on the inlet of the nacelle. There are,however, drawbacks to the use of this anti-ice system. The exhaust fluidremoved through the exhaust holes is typically flowed into a chamberwithin the nacelle. The exhaust fluid exits the nacelle through a ventand flows into the external medium flowing about the nacelle. Flowingthe exhaust gases through the nacelle structure may lead to overheatingof the nacelle in the vicinity of the chamber.

Further, the fluid exiting the vent tends to be held against the outerbarrel skin by the external medium flowing past the outer skin (the freestream). This may lead to overheating of the nacelle surface downstreamof the vent. Overheating of the nacelle is especially significant if thenacelle is made from lightweight composite materials. These compositematerials are typically bonded together with an adhesive having a lowermelting temperature than traditional metallic materials. The rate oftemperature decay downstream of the vent is dependent upon the size ofthe vent and a blowing parameter. Large values for the area of the ventresult in a low rate of decay of temperature downstream of the vent.Small values of the blowing parameter, defined as the ratio of exhaustexit velocity to free stream velocity, also result in a low rate ofdecay of temperature downstream of the vent. The affects of vent sizeand blowing parameter or the rate of decay of temperature arecumulative.

The vent also introduces aerodynamic penalties in the form of increaseddrag of the nacelle when the anti-ice system is shut off. The dragcaused by the vent is dependent upon the size of the vent and the axiallocation of the vent. The larger the exit area of the vent or the closerthe vent is to the leading edge of the nacelle, the greater the dragattributable to the vent.

A solution to the overheating of the nacelle caused by the exhaust flowthrough the nacelle is to duct the exhaust flow from the cavity to thevent. Ducting the exhaust fluid through the nacelle structure reducesthe likelihood of overheating of the nacelle in the vicinity of thechamber, but also increases the size and weight of the nacellestructure. Attempts to decrease the bulk of the ducts by moving theexhaust vent forward, and thereby shortening the length of the duct,increases the aerodynamic drag of the vent. In addition, the duct doesnot prevent overheating of the nacelle surface downstream of the vent.

The above art notwithstanding, scientists and engineers under thedirection of Applicants' Assignee are working to develop efficientanti-icing systems for the nacelle of a gas turbine powerplant.

DISCLOSURE OF THE INVENTION

The invention is predicated in part upon recognition by the Applicantsof the availability within the cavity of a dynamic pressure head toincrease the velocity of the exhaust fluid exiting the cavity. Prior artexhaust means rely upon the static pressure differential between thecavity and the environment external to the cavity to produce the exhaustflow. This resulted in low exit velocities for the exhaust fluid and,consequently, large exhaust vents to provide an adequate, volumetricflow rate of exhaust fluid out of the cavity. The combination of smallblowing parameter and large vent size results in overheating of thenacelle external surface. Using the dynamic pressure head available fromthe circumferentially flowing body of fluid increases the exit velocityand decreases the size of the exhaust vent required.

According to the present invention, an anti-ice system for the leadingedge of a nacelle includes a cavity bounding a body of fluid having acircumferentially directed component of velocity and means to exhaustthe fluid outboard of the nacelle, wherein the exhaust means is adaptedto turn the exhaust flow such that it is substantially normal to thedirection of flow of ambient air external to the cavity, and wherein theexhaust means is adapted to use the dynamic pressure head within thecavity to urge the exhaust flow to exit the cavity with sufficient exitvelocity to penetrate into the medium external of the nacelle.

According to a specific embodiment of the present invention, the exhaustmeans includes a plurality of aerodynamic vanes disposed within thecavity and on the radially outer portion of the nacelle, a plurality ofvent slots disposed between adjacent vanes, an upstream fairing, and adownstream fairing, wherein the plurality of vanes are adapted to turn aportion of the circumferentially flowing body of fluid such that theturned portion of the fluid passes through the slots and out of thecavity. Each of the vanes extend in a direction normal to the principaldirection of flow within the cavity. Each of the slots is disposedadjacent to, parallel with, and upstream of, relative to the principledirection of flow within the cavity, one of the vanes.

A principal feature of the present invention is the plurality ofaerodynamically, bluntly shaped vanes disposed within the cavity.Another feature is the plurality of exhaust slots. A further feature isthe fairings located upstream and downstream of the vanes and slots.

A primary advantage of the present invention is the rate of decay oftemperature downstream of the exhaust vent as a result of the exhaustfluid ejection angle, the blowing parameter of the exhaust means, andthe size of the exhaust slots. The exhaust fluid is ejected through theplurality of small exhaust slots at an ejection angle substantiallynormal to the free stream flow direction and with sufficient velocity topenetrate the external medium flowing over the exhaust means. Thepenetration of the free stream creates a separation between the hotexhaust fluid and the nacelle and prevents reattachment of the exhaustfluid to the surface of the nacelle. Another advantage is the level ofefficiency of the exhaust means as a result of the utilization of thedynamic pressure head within the cavity to produce the exhaust flow. Theaerodynamic vanes interact with the fluid in the cavity to redirect aportion of it through the slots, rather than relying upon staticpressure only to urge the fluid out of the cavity. A further advantageis the reduced aerodynamic drag of the vent, relative to prior artvents, as a result of the reduced size of the vent. The reduced ventsize is a result of the increased velocity of the exhaust fluid ejectedfrom the cavity. Another advantage is the tolerance of the exhaust meansfor non-circumferential flows within the cavity, such as helical flows,as a result of the bluntly shaped vanes. A further advantage is theminimal flow losses within the cavity as a result of the fairings whichprovide smooth changes in flow area and block flow separation fromoccurring. Minimizing flow losses within the cavity maximizes thepressure differential available to eject the exhaust fluid from thecavity.

Although the invention is shown and described as means to exhaust fluidfrom a cavity having a circumferentially flowing body of fluid, theinvention is equally applicable for cavities having other types of flow,such as a spiraling or helical flow about a circumferential axis throughthe annular cavity.

The foregoing and other objects, features and advantages of the presentinvention become more apparent in light of the following detaileddescription of the exemplary embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a gas turbine engine and a nacelle.

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1 to showa cross-sectional view of a plurality of exhaust vanes.

FIG. 3 is a perspective view, partially cut away, of the exhaust means.

FIG. 4 is a view taken along line 4--4 of FIG. 1, and shows theplurality of exhaust slots.

FIG. 5 is a cross-sectional view of an exhaust vane.

FIG. 6 is a plot of temperature decay as a function of anon-dimensionalized distance downstream of a vent.

BEST MODE FOR CARRYING OUT THE INVENTION

A gas turbine powerplant 12 is illustrated in FIG. 1. The powerplant 12is disposed about a longitudinal axis 14 and includes an axial flow gasturbine engine 16 and a nacelle 18. The gas turbine engine 16 includesan axially extending flowpath 22, a compression section 24, a diffusercase 25, a combustion section 26, and a turbine section 28. Incomingfluid passes through the nacelle 18 and into the compression section 24of the gas turbine engine 16. Working fluid is compressed within thecompression section 24 which results in an increase in momentum of theworking fluid and an increase in temperature of the working fluid. Fromthe compression section 24 the working fluid flows through the diffusercase 25 and into the combustion section 26 where fuel is mixed with theworking fluid and the mixture is combusted. The products of combustionare then expanded through the turbine section 28 and then flowed out ofthe gas turbine engine 16.

The nacelle 18 extends forward of the gas turbine engine 16 and definesan inlet 32 for incoming fluid. The nacelle 18 includes an inner barrel34, an outer barrel 36, a bulkhead 38, and an inlet shell 42. The innerbarrel 34 defines a radially outer flow surface 44 for a portion of theflowpath of the gas turbine engine 16. The outer barrel 36 defines anouter flow surface 46 for the external medium flowing about the gasturbine engine 16. A radial separation between the inner barrel 34 andthe outer barrel 36 defines an annular chamber 48 therebetween. Theinlet shell 42 is the leading edge for the nacelle. The inlet shell 42and the bulkhead 38 bound an annular shell cavity 52.

The powerplant 12 includes an anti-ice system 54. The anti-ice system 54includes a conduit 56, a nozzle 58, and exhaust means 62. The conduit 56extends between the diffuser case 25 of the gas turbine engine 16 andthe shell cavity 52. Although not shown in FIG. 1, the conduit 56 mayinclude a variety of valves and flow regulators to control the flow offluid through the conduit. The diffuser case 25 is a source of hightemperature, high pressure fluid. The nozzle 58 is attached to an outletend 64 of the conduit 56 and extends within the shell cavity 52. Theexhaust means 62 provides fluid communication between the shell cavity52 and the external medium flowing about the powerplant 12. The exhaustmeans 62 is disposed on the radially outer portion and axiallydownstream end of the inlet shell 42. Although the exhaust means 62 maybe located at various circumferential positions around the inlet 52,stability of the pressure field of the medium external to the exhaustmeans 62 may require consideration. As is well known, in aircraftapplications a position 180° from top dead center is a region of stablepressure. Therefore, for aircraft application it is suggested that thecircumferential location of the exhaust means 62 be 180° from top deadcenter.

Referring now to FIGS. 2-4, the exhaust means 62 is locatedcircumferentially adjacent to and upstream of, relative to the flowwithin the shell cavity 52, the nozzle 58. This location removes fluidwhich has transferred heat as it travelled around the circumference ofthe shell cavity 52. In effect, the exhaust means 62 removes relativelycool exhaust fluid from the shell cavity 52.

The exhaust means 62 includes a plurality of vanes 66, a plurality ofexhaust slots 68, an upstream fairing 69, a downstream fairing 70, andan axially adjacent wall 72. The plurality of exhaust vanes 66 extendradially inward from the inlet shell 42 and into the shell cavity 52.The plurality of vanes 66 also extend axially. Each vane 66 isaerodynamically shaped to engage the body of fluid flowingcircumferentially within the shell cavity. As shown in FIG. 5, the vaneshave a rounded leading edge 74 which adapts the vanes to engage withfluid flowing at off angles relative to the circumferential direction.The roundness of the vane leading edge 74 is defined by the ratio ofleading edge 74 diameter d, measured as shown in FIG. 5, to maximumthickness t, measured in the circumferential direction. To accommodateoff angle flow within the shell cavity 52, it is suggested that theratio d/t be between 0.4 and 0.6. The shape of the vanes 66 is alsodependent upon the ratio of maximum thickness t to vane length l. It issuggested that the ratio t/l be between 0.3 and 0.5. While the vanes 66have been described with respect to various ratios of length l,thickness t, and leading edge diameter d, it will be appreciated bythose skilled in the art that the values of these ratios vary withapplication and the suggested ranges of values may be transgressedwithout departing from the scope of the invention.

Each of the exhaust vanes 66 is circumferentially adjacent to anddownstream of, relative to the direction of flow of the body of fluidwithin the shell cavity 52, one of the plurality of exhaust slots 68.The plurality of exhaust slots 68 extend radially through the inletshell 42 and provide means of communication between the shell cavity 52and the external medium. The exhaust slots 68 are circumferentiallyadjacent and extend in an axial direction as shown in FIG. 4.

The upstream fairing 69 provides a smooth transition from the flowsurface of the inlet shell 42 to the vanes 66. The downstream fairing 70provides a smooth transition from the vanes 66 back to the flow surfaceof the inlet shell 42. The fairings 69, 70 eliminate abrupt changes incross-sectional area within the shell cavity 52. In accordance withconventional fluid mechanics, the fairings 69, 70 produce smooth flowlines over the exhaust means 62 and uniformly distribute the exhaustflow through the exhaust slots 68. Without the fairings 69, 70, anunequal distribution of the flow through the slots would occur. Unequaldistribution implies a flow separation, with corresponding flow lossesand inefficiencies, within the shell cavity 52.

FIGS. 1-4 show exhaust means having five (5) vanes and six (6) slots.The quantity and size of the vanes and slots is dependent upon the flowcharacteristics of the shell cavity and the flow rate of the source ofhigh temperature, high pressure fluid. The quantity and size of thevanes and slots must be sufficient to prevent over pressurization of theshell cavity when subjected to the maximum output of the source of hotfluid. In addition, the quantity and size of the vanes and slots mustpermit sufficient pressurization of the shell cavity to produce adequateflow velocity through the slots and penetration of the external flow.Temperature considerations dictate having multiple small slots toincrease the rate of temperature decay of the fluid exiting each slot. Aplurality of small slots produce a higher rate of temperature decay thana single large opening.

During operation of the gas turbine engine 16, compressor dischargefluid is removed from the diffuser case 25. The combustion section 26 isa source of high pressure, high temperature fluid (relative to theexternal medium) which may be used as heating fluid in the anti-icesystem. The fluid is then flowed through the conduit 56, is ejectedthrough the nozzle 58 and into the shell cavity 52. The conduit 56provides means to flow the high temperature, high pressure fluid intothe shell cavity 52. As shown in FIGS. 1 and 2, the fluid is ejectedfrom the nozzle 58 in a direction tangential to the circumferentialdirection. The nozzle 58 provides means to inject fluid into the shellcavity 52 to generate a flowing body of fluid. The high pressure fluidexits the nozzle 58 with sufficient velocity to entrain the body offluid within the shell cavity 52 and produce a body of fluid having aprincipal component of velocity directed in the circumferentialdirection.

Exhaust means 62 is necessary to prevent over-pressurization of theshell cavity 52. The exhaust means 62 removes fluid from the radiallyoutermost and axially downstream section of the shell cavity 52. Theexhaust vanes 66 turn a portion of the flow such that the exhaust fluidis ejected from the cavity with an ejection angle α defined by an theangle formed by the direction of the exhaust flow and the externalsurface of the nacelle, equal to 90°. Redirecting the flow to adirection substantially normal to the direction of flow of the externalmedium maximizes the rate of decay of temperature downstream of theexhaust slots 68.

The exhaust vanes 66 utilize the dynamic pressure head within the shellcavity in addition to the static pressure head to urge the fluid throughthe exhaust slots 68. The combination of dynamic and static pressurehead produce an exhaust flow velocity sufficient to permit the exhaustflow to penetrate the external medium flowing by the exhaust slots 68.By penetrating the external medium a separation is created between thehot exhaust fluid and the nacelle 18. The separation region is a lowpressure region which pulls in low temperature external medium. As aresult, the exhaust fluid mixes with external medium rather thanreattaching to the surface of the nacelle 18. In addition, the highvelocity of the exhaust flow permits the total area of the exhaust slots68 to be reduced relative to prior art exhaust vents.

FIG. 6 is an illustration of test results of measured temperature decayas a function of distance downstream. Temperature decay is the ratio ofthe difference between surface temperature T and free stream temperatureT_(oo) to the difference between vent exit temperature T_(o) and freestream temperature T_(oo). Distance downstream is shown as a ratio ofx/D, where x is the actual distance and D is the effective diameter ofthe vent. Effective diameter D is the diameter of a circle equal in areato the vent. In this way the downstream distance is non-dimensional andthe size of the vent is parametrically accounted for. The vanes providemeans to turn a portion of the fluid flowing within the shell cavity toa direction substantially normal to the flow direction of the externalmedium flowing over the surface of the inlet shell in the vicinity ofthe exhaust means. Turning the exhaust flow normal to the free streamdirection increases the rate of decay of temperature along the surfacedownstream of the exhaust means, as shown in FIG. 6. Curve "A"represents a circular vent with an ejection angle α.sub. A =35° and ablowing parameter β_(A) =0.4. The results used in curve "A" are forcomparison purposes and are taken from "Film Cooling with Injectionthrough Holes: Adiabatic Wall Temperatures Downstream of a CircularHole", ASME Paper No. 68-GT-19, authored by Goldstein, Eckert andRamsey. Curve "B" represents test results for a circular, vent with anejection angle α_(B) =90° and a blowing diameter β_(B) =0.4. Curve "C"represents test results for a slotted vent with an ejection angle α_(C)=90° and a blowing parameter β_(C) =0.4. Although turning the exhaustflow from a substantially tangential direction (α_(A) =35°) to adirection normal to the free stream (α_(B) =α_(C) =90°) increases therate of temperature decay dramatically, improvement in rate oftemperature decay may also be produced by ejection angles as low asα=60°.

The vanes also provide means to use the dynamic pressure head within thebody of fluid flowing in the shell cavity. Using the dynamic pressurehead in addition to the static pressure head increases the exit velocitythrough the exhaust means and decreases the size of the vent requiredrelative to prior art exhaust vents. Higher exit velocity produces ahigher blowing parameter β. Increasing the blowing parameter β increasesthe rate of temperature decay, as shown in FIG. 6. Curves "D" and "E"represent the results for circular and slotted vents, respectively,having a blowing parameter β_(D) =β_(B) =0.7. Curve "F" represents theresults for a circular vent having a blowing parameter β_(F) =1.2.

Smaller vent size D results in a larger value for x/D at a givendistance X downstream of the exhaust means. Therefore, smaller ventsproduce increased rates of temperature decay for a given ejection angleα and blowing parameter β. The size of the vents or slots may be reducedby increasing the exit velocity, and thereby reducing the size of a slotneeded to produce a required volumetric flow rate, and by increasing thenumber of slots, and thereby reducing the size of each slot.

It should be noted that the vanes 66 and slots 68 are shown in FIGS. 1-4as extending in a direction normal to the circumferential direction,i.e. they extend axially. The vanes 66 are oriented in this direction tomaximize the efficiency of the interaction between the body of fluidflowing within the shell cavity 52 and the exhaust means 62. Theprincipal component of velocity for the body of fluid is directed in thecircumferential direction. During operation, however, differing thermalboundary layers may build up around the internal flow surfaces of theshell cavity 52 and may result in the body of fluid within the cavity 52having a non-circumferential component of velocity. The blunt shape ofthe vanes 66 adapt the exhaust means 62 to accommodatenon-circumferential flow and to urge this type of flow into the exhaustmeans 62. In some situations, it may be desirable to angle the nozzle58, or a plurality of nozzles 58, to induce a spiralling or helical flowwithin the shell cavity 52. For such other flow conditions the vanes 66and slots 68 may be canted relative to the longitudinal axis 14. Cantingthe vanes 66 may provide more efficient engagement of the exhaust means62 with the non-circumferential flow within the shell cavity 52. Fortypical vanes, efficient engagement anticipates vanes extending in adirection substantially normal to the principal direction of flow thevanes are engaged with.

It should also be noted that, although the diffuser case 25 wasdescribed as the source of high temperature, high pressure fluid, othersections of the powerplant 12 may function as a source of hightemperature, high pressure fluid. For instance, the compression section24 may be used. Additionally, the exhaust means 62, though shown anddescribed as ejecting fluid radially outboard of the nacelle, may bedisposed to eject fluid radially inward such that the exhausted fluid isrecycled into the gas turbine engine 16.

Although the invention has been shown and described with respect toexemplary embodiments thereof, it should be understood by those skilledin the art that various changes, omissions, and additions may be madethereto, without departing from the spirit and scope of the invention.

We claim:
 1. A powerplant for an aircraft, the powerplant including alongitudinal axis, a nacelle disposed about the axis and defining anaxial flowpath, the nacelle having an inner barrel defining a radiallyouter flow surface for a portion of the flowpath, an outer barreldisposed radially outward of the inner barrel and defining a flowsurface for an external medium flowing over the nacelle, a forwardbulkhead extending between the axially forward ends of the inner barreland outer barrel, an annular inlet shell extending forward of theforward bulkhead, and an anti-ice system, wherein the anti-ice systemincludes:a source of heating fluid, the source producing a fluid havinga temperature and pressure greater than the temperature and pressure ofthe fluid medium external to the gas turbine engine; a cavity bounded bythe inlet shell and the forward bulkhead; a conduit extending from thesource to the cavity and defining a flowpath for heating fluid, theconduit having an outlet; a nozzle disposed within the cavity andattached to the outlet of the conduit, the nozzle defining means toinject heating fluid into the cavity and to generate a body of fluidflowing within the cavity with a circumferentially directed component ofvelocity; a plurality of exhaust slots disposed on the radially outerportion of the inlet shell; a plurality of aerodynamically shapedexhaust vanes, each of the exhaust vanes disposed on the inlet shell,extending radially into the cavity, and being adjacent to and extendingparallel to one of the plurality of exhaust slots, the exhaust vanesadapted to engage the fluid flowing within the cavity; and wherein theplurality of exhaust vanes engage the fluid flowing within the cavity toturn a portion of the fluid to a direction substantially normal to thedirection of flow of the medium external to the nacelle and to urge theportion of fluid to flow through the plurality of exhaust slots.
 2. Ananti-icing system for a powerplant having a longitudinal axis, a sourceof heating fluid, and a nacelle defining a radially outer boundary forthe power plant, the nacelle having a cavity disposed about the axis,the anti-icing system including:means to flow heating fluid from thesource to the cavity; means to inject heating fluid into the cavity, theinjection means injecting fluid such that a body of fluid flowing withinthe cavity is generated; and means to exhaust fluid from the cavity intoa medium external of the nacelle, the exhaust means engaging the fluidflowing within the cavity to turn a portion of the fluid to a directionsubstantially normal to the direction of flow of the medium external ofthe nacelle and to eject the fluid being exhausted at a flow velocitysufficient to penetrate the external medium.
 3. The anti-icing systemaccording to claim 1, wherein the nacelle defines a fluid inlet for thepowerplant, wherein the cavity is annular and disposed about the inlet,and wherein the injection means injects fluid such that the body offluid flowing within the cavity flows with a circumferentially directedcomponent of velocity.
 4. The anti-icing system according to claim 3,wherein the exhaust means includes:an exhaust slot disposed on theradially outer portion of the inlet shell, an aerodynamically shapedexhaust vane, the exhaust vane disposed on the inlet shell, extendingradially into the cavity, and being adjacent to and extending parallelto the exhaust slot, the exhaust vane adapted to engage the body offluid flowing within the cavity and to urge a portion of the fluidthrough the exhaust slot.
 5. The anti-icing system according to claim 4,further comprising a fairing disposed upstream relative to the directionof flow of the body of fluid within the cavity, of the exhaust vane andadapted to block flow separation from occurring.
 6. The anti-icingsystem according to claim 4, further comprising a fairing disposeddownstream relative to the direction of flow of the body of fluid withinthe cavity, of the exhaust vane and adapted to block flow separationfrom occurring.
 7. The anti-icing system according to claim 4, whereinthe exhaust vane has a maximum thickness t, measured in thecircumferential direction, and a rounded leading edge having a leadingedge diameter d, and wherein the ratio d/t is between 0.4 and 0.6. 8.The anti-icing system according to claim 4, wherein the exhaust vane hasa maximum thickness t, measured in the circumferential direction, and alength l, measured in the radial direction, and wherein the ratio t/l isbetween 0.3 and 0.5.
 9. The anti-icing system according to claim 3,wherein the exhaust means further includes means to turn the portion ofthe fluid such that the fluid is ejected from the cavity in a directionforming an angle α with the surface of the nacelle, the angle α beinggreater than or equal to 60°.
 10. The anti-icing system according toclaim 3, wherein the fluid being ejected has a blowing parameter β,defined as the ratio of the magnitude of velocity of the ejected fluidto the magnitude of velocity of the external medium flowing over thenacelle, and wherein the exhaust means further includes means to ejectfluid such that the blowing parameter β is greater than or equal to 0.411. The anti-icing system according to claim 4, wherein the exhaust vaneand exhaust slot extend in a direction normal to the direction of flowof the body of fluid within the cavity.
 12. The anti-icing systemaccording to claim 4, wherein the exhaust vane and exhaust slot providemeans to eject the fluid in a radially outward direction.